Field of the Invention
The present invention relates to a secondary brake for commercial vehicles. More particularly, the present invention relates to a secondary brake for commercial vehicles using magnetorheological (MR) fluid that has high braking output per unit volume or unit weight because large braking power can be provided using a small amount of fluid, is possible to be manufactured in a reliable and simple structure, and improves marketability of the commercial vehicle.
Description of Related Art
Generally, since commercial vehicles are very heavy in weight, a fade phenomenon (a phenomenon in that braking power is decreased by deteriorated contact between a drum and a lining due to burning and deformation of the brake drum of a vehicle by frictional heat) caused by overheating generated when a primary brake (a wheel brake) is excessively used may lead to an accident of the commercial vehicle.
Accordingly, secondary brakes are necessarily applied in most of the commercial vehicles. The secondary brakes are divided into an exhaust brake, an engine brake, and a fluid-type retarder brake, etc., according to a control method.
Among them, the fluid-type retarder brake is widely used for heavy-duty trucks because it has the greatest brake torque. The fluid-type retarder brake exhibits 15% performance of a brake torque of a common primary brake, and braking ability that usually can brake the trucks on a road having about 9% gradient without braking the primary brake.
However, while the primary brakes exhibit a response characteristic of 0.5 second or less, the fluid-type retarder secondary brakes exhibit a response characteristic about 0.9˜1.5 seconds, so that the fluid-type retarder secondary brakes have a much slower response speed than the primary brakes.
That is, as illustrated in FIG. 1, since a transmission output shaft 10 and a rotor 20 are integrally rotated while the fluid-type retarder brake is not operated, a rotational resistance due to viscosity resistance of the fluid is generated when operating fluid is present in a space between the rotor 20 and a stator 30.
Accordingly, in order to prevent rotational resistance, the operating fluid is stored in a separate reservoir and fills up the operating space by means of an oil pump 40 when the retarder brake is operated, thereby response time for operating of 0.9˜1.5 seconds is required, thus the retarder brake has difficulty in being used during an emergency and has a tendency of being braked easily only in a situation in which slow braking is required.
Although the braking torque is output by adjusting fluid density as describe above, since viscosity characteristic of the fluid is constant, in order to adjust a magnitude of the braking torque output, the density of the operating fluid filled into the space between the rotor 20 and the stator 30 (an amount of fluid being filled into the space) should be controlled as required.
Accordingly, braking is operated with a somewhat slow and sluggish feel in a braking situation requiring a quick response or in an interlocking operation with the primary brake.
Meanwhile, the fluid-type retarder brake has a very large outer size and a heavy weight compared to an output performance and is expensive considering the output performance, due to the components including the rotor 20, the stator 30, an operating fluid reservoir 50, and the oil pump 40, etc., so that there is a problem of the vehicle purchase price being increased.
Meanwhile, since the application of the secondary brake is compulsory depending on laws by each country for a reason of commercial vehicle safety, it is required in future that competitive power of a product at a relatively low price comparable to the output performance is secured.
An electronic brake system is actively applied recently by digitalization of the primary brake control, and active control of the primary and secondary brakes is attempted in various situations.
However, since the fluid-type retarder brake has a disadvantage in that feeling of control cannot be easily improved compared to the braking performance because of a slow control response due to the fluid characteristic, there is an insufficient feel of braking in an electronically controlled interlocking operation with the primary brake.
As a result, although the fluid-type retarder brake has the highest braking performance as the secondary brake, it has a disadvantage in that it is bulky and weight is increased because it uses fluid.
Accordingly, a retarder type secondary brake that exhibits large braking torque with small volume and a quick control characteristic is required.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.